1. Field of the Invention
This invention generally relates to an optical semiconductor device and a controlling method of the optical semiconductor device.
2. Description of the Related Art
Generally, a wavelength-tunable semiconductor laser has a structure in which a laser diode and a thermistor are mounted on a mount carrier and the mount carrier is mounted on a temperature control device (TEC). The wavelength-tunable semiconductor laser has a gain for a laser emission and can select a wavelength of the laser. There are some methods of selecting a wavelength. For example, the methods include a method of changing a resonant wavelength of loss or gain by changing a refractive index or angle of a diffractive grating or an etalon provided in a laser cavity. And the methods include a method of changing a resonant wavelength of the laser cavity by changing an optical length in the laser cavity (refractive index or a physical length of the laser cavity).
The method of changing the refractive index has an advantage in reliability or manufacturing cost, because a mechanical operating portion like the method of changing the angle or length is not necessary. The refractive index changing method includes changing a temperature of an optical waveguide, changing a carrier density in the optical waveguide by providing a current, and so on. A semiconductor laser having a Sampled Grating Distributed Reflector (SG-DR) is supposed as a wavelength tunable semiconductor laser that changes a temperature of an optical waveguide, where the SG-DR has a wavelength selection function.
In this semiconductor laser, if a reflection spectrum of a plurality of SG-DR regions (reflection region) is controlled preferably, a predetermined wavelength can be selected with a vernier effect. That is, this semiconductor laser emits a laser light at a wavelength where reflection peaks of two SG-DR regions are overlapped with each other. It is therefore possible to select the lasing wavelength by controlling each of the reflection peaks of the SG-DR regions.
Generally, a heater is provided on a surface of one of the SG-DR regions. It is possible to change the temperature of an optical waveguide of the SG-DR region where the heater is provided, with heat generated by the heater. As a result, a refractive index of the optical waveguide is changed. Accordingly, it is possible to select a reflection peak wavelength of the SG-DR region where the heater is provided, by controlling the heating value of the heater.
It is possible to control the lasing wavelength to be a desirable one by controlling the refractive index of the optical waveguide of the SG-DR segments, with use of a temperature control device providing heat to whole of a semiconductor laser. In this case, it is necessary to control the temperature of the semiconductor laser accurately. And so, Japanese Patent Application Publication No. 11-186645 (hereinafter referred to as Document 1) discloses a method of feedback control according to a detection result of a resistance of a thermistor arranged on a predetermined position of the temperature control device.
In Document 1, the thermistor is arranged near a semiconductor laser chip not having a heater. Therefore, the thermistor can detect a temperature accurately. However, in the conventional art mentioned above, the heater is provided on a surface of the SG-DR region. The thermistor arranged near the SG-DR region is subjected to heat from the heater and from a wire connected to the heater, when heating value of the heater is large. In this case, there is generated a temperature detection error in the thermistor. It is therefore difficult to control the lasing wavelength of the semiconductor laser accurately.